How to setup shared service business unit in Purchasing

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When I am creating a requisition in BU A, I want it to be procured by either BU A or BU B and hence my expectation is it (Requisition created under requisitioning BU A) should be available under both A & B in Process Requisition screen. But in this case, it is only available under A as I have done buyer assignment rules and I think as per rules it is assigning procurement BU which I have assigned in buyer assignment. So even if I set another rule by only changing BU B and keeping other parameters same, still it